Polyacetylene is prepared as disclosed in the Journal of Polymer Science, Volume 12, pages 11 through 20, Shirakawa, et al (1974), and Trans. Faraday Society, Volume 64, pages 823 through 828, Berets, et al (1968). The disclosures of these papers are incorporated herein by reference.
Polyacetylene has valuable electrical properties for a wide variety of uses. However, when polyacetylene is prepared, within a short time after its preparation, the polyacetylene becomes brittle and also loses a portion of its ability to acquire enhanced electrical conductivity properties when doped. Even when a polyacetylene powder is prepared, the ability of such powder to acquire enhanced electrical conductivity decreases after a short period of time and the powder itself becomes modified so that the preparation of formed articles from the powder becomes difficult. One possible explanation for the loss of ability to acquire enhanced conductivity and the embrittlement of a polyacetylene formed material, such as a film, is that the polyacetylene, when prepared at -78.degree. C., is in the form of cis-polyacetylene. However, it is known that cis-polyacetylene, although generally considered stable at temperatures of from about -78.degree. C. to 0.degree. C., does isomerize slowly, even at -78.degree. C., to trans-polyacetylene. At temperatures in excess of 0.degree. C., isomerization of cis-polyacetylene to transpolyacetylene is accelerated. During this conversion, free radicals may be formed which may crosslink or otherwise react with available oxygen. The reaction with available oxygen is believed to contribute to the embrittlement of, for example, a polyacetylene film by the formation of carbonyl and hydroxyl groups. These groups disrupt the conjugation of the polyacetylene double bonds and thereby decrease the ability of the polyacetylene to acquire enhanced electrical conductivity. Whenever cis-polyacetylene is isomerized to transpolyacetylene, there will always be the formation of free radicals due to the isomerization mechanism. A discussion of the preparation of polyacetylene films and the isomerization of such films is set forth in the Journal of Polymer Science, Volume 12, pages 11 through 20, Shirakawa, et al (1974).
Embrittlement of a cis-polyacetylene film or formed article can be delayed by storing the film or formed article at a low temperature (-78.degree. C. to 0.degree. C.) under an inert gas such as nitrogen, argon or helium.
Although it is known that the cis-polyacetylene is more flexible than the trans-polyacetylene, the trans-polyacetylene has greater intrinsic electrical conductivity properties and the trans- form is thermodynamically more stable. The free radicals which may be formed during isomerization of cis- to trans polyacetylene also trap oxygen and reduce the electrical conductivity potential of the polyacetylene (whether cis- or trans- if oxygen is present because it is believed that these free radicals form carbonyl and hydroxyl groups). Although, the state of the art is still such that the formation of these free radicals cannot be eliminated, if the presence of oxygen can be eliminated, then an aggravation of the results of free radical formation can be avoided. Thus, the problems of embrittlement and loss of electrical conductivity potential can be alleviated.
The previous practice of avoiding embrittlement involved preparation of cis-polyacetylene and storage of the cis-polyacetylene at low temperatures of from -78.degree. C. to 0.degree. C. under vacuum or an atmosphere of an inert gas. Such procedures are cumbersome in any practical ambient environment. Therefore, the utility of polyacetylene in applications requiring electrical conductivity is severely limited by the use of those procedures.
Any other approach to the aforesaid problem of the effects of oxygen must take into consideration the affinity of polyacetylene for oxygen. Thus, any material which would remove oxygen from the system must compete with the polyacetylene for the removal of such oxygen and must have a greater affinity for oxygen than the polyacetylene. Stated otherwise, any material which would remove oxygen must be able to compete successfully with polyacetylene for the oxygen present.
The Journal of the American Chemical Society, Volume 46, pages 2639 through 2647, L. F. Fieser (1924), discloses certain aqueous solutions which are useful as absorbents for oxygen in gas analysis. These solutions comprise an anthraquinone salt, a base and a reducing agent. This article also discloses a potassium pyrogallate solution used for comparative purposes and also the use of pyrogallol and the use of a hyposulfite solution. However, this article does not deal with the problem of two substances competing for oxygen and the article is directed to oxygen absorbents in gas analysis and does not teach or suggest the problems set forth herein or any polyacetylene material.
When cis-polyacetylene is available and it is desired to isomerize the cis-polyacetylene, partly or wholly, to trans-polyacetylene, the problem of the presence of oxygen is also encountered in that if oxygen is present, then because the isomerization of cis-polyacetylene to trans-polyacetylene may involve the formation of free radicals which can react with the oxygen to form carbonyl and hydroxyl groups and thereby to adversely affect the electrical conductivity potential, it is important that oxygen be excluded from the polyacetylene when it is being isomerized from the cis form to the trans form. Additionally, the presence of oxygen, during isomerization, leads to more severe embrittlement of the resultant trans-polyacetylene. Oxygen must also be excluded from trans-polyacetylene because of the adverse effect of oxygen on the trans form which causes further embrittlement of the film and reduction of the electrical conductivity potential. The present method of excluding oxygen, namely accomplishing isomerization under an inert atmosphere, is unattractive.
It is an object of this invention, therefore, to reduce polyacetylene crosslinking and embrittlement.
Another object of this invention is to provide a process for substantially preventing oxygen from contacting polyacetylene by providing a material which will successfully compete with the polyacetylene for the available oxygen.
Still another object of this invention is to provide a process for maintaining the electrical conductivity potential of polyacetylene.
A further object of this invention is to provide a process for isomerizing cis-polyacetylene to transpolyacetylene.
Other objects and advantages will become apparent from the following more complete description and claims.